This invention relates to a system for recording electrical waveforms and more particularly to such a system employing a charge transfer device.
Prior art waveform recorders are bulky, expensive items which typically have bandwidths of about 1 gigahertz or less.
The fastest recorders of nonrepetitive electrical waveforms are oscilloscopes, which operate on the principle of deflecting on electron beam. A common way to achieve fast response is to compromise the sensitivity of the oscilloscope. Compensation for the loss in sensitivity can sometimes be made by employing amplification in the electronics, which is difficult and expensive for fast pulses.
Digital recorders of nonrepetitive waveforms are even more expensive and generally have less bandwidth than state-of-the-art oscilloscopes.
For repetitive signals, sampling scopes offer better temporal resolution than oscilloscopes. One reason sampling scopes are used is to obtain temporal resolution of the waveform, even if that requires the added inconvenience of sampling many repetitions of the waveform. If superior temporal resolution were available, many applications of the expensive sampling scopes would be obviated.
Present sytems for fast detecting and recording of photons are often bulky, like streak cameras, or else are typically limited to a bandwidth of 1 GHz or less. Active electronic detection systems like CCD's must have cycle times shorter than the temporal resolution desired.
Travelling surface acoustic waves (SAW) are generated by applying an RF signal to interdigitated contacts of suitable size located on a surface with piezoelectric properties. The interaction of the travelling SAW with the piezoelectric medium produces a potential wave which travels with the acoustic wave. By arranging a semiconductor to be in close proximity to the piezoelectric medium, the potential wave can be made to extend into the semiconductor, resulting in travelling potential wells in the semiconductor. Charge carriers can be injected into these wells by a source of carriers such as a signal generator, and the carriers are then transported in the wells by the SAW. The presence of a biased, conducting field plate over the charge transfer region is used to deplete the semiconductor in that region. Minority charge carriers which are injected into the SAW and beneath the field plate are transported in the travelling wells at the velocity of the SAW. The majority carriers are repulsed by the field plate. The minority charge carriers are retained beneath the field plate, rather than conducting away in a direction perpendicular to the propagation vector of the SAW, owing to the attractive potential established by the field plate. (The field plate thus behaves much like an MOS gate in a CCD.) The carriers in neighboring travelling wells do not mingle, so long as the wells are not overfilled. The conventional method of collecting the charges involves collecting the charge with a detector which is placed directly in the path of the SAW. The disadvantage of this approach is that an injected 10 GHz signal must be read and recorded with 10 GHz electronics. Travelling SAWs have been used to transfer electronic charges along or near the surface of piezoelectric materials, the resulting devices being referred to as charge transfer devices (CTD's). Such devices have been disclosed in the article "A Monolithic SAW-Charge Transfer Device" by N. A. Papanicolaou and H. C. Lin in Remote Sensing of Earth from Space: Role of "Smart Sensors", edited by Roger A. Breckenridge, Vol. 67 of Progress in Astronautics and Aeronautics, pp. 325-351 (1979); in the Ph.D. dissertation entitled "A Monolithic Surface Acoustic Wave Charge Transfer Device" by Nicolas A. Papanicolau, for the Electrical Engineering Dept., University of Maryland, 1979; and in the pending U.S. Patent application of the same title, all whose disclosures are herein incorporated by reference.